This invention relates to a water heater the type which in use, has a continuous flow of water therethrough, the water being heated by an electrical heating means as it flows. Such a heater will hereinafter be referred to as an instantaneous water heater.
Previously, such water heaters have been provided with an electrical heating means which delivers heat at a fixed rate to the water flowing, and the temperature of the outflowing water is controlled by adjusting the flow rate of the water.
However, in for example, a domestic shower heater application, it is preferred for a user to be able to adjust the rate at which the heat is dissipated to the flowing water, to control the temperature of the outflowing water without having to adjust the flow rate. Thus various proposals have been put forward to achieve this.
First, it has been proposed to rapidly switch the electrical heating means on and off, e.g. at the mains frequency, fifty times per second (in the U.K.), using a solid state device such as a triac, and to vary the number of "on" cycles each second or vary the "on" time relative to the "off" time by changing the frequency of switching or relative durations of the "on" and "off" times, whereby the power dissipated by the heater over a given period, such as each second, can be varied. This technique is known as "burst firing".
An example of a circuit which may be used for such an arrangement is described in U.K. Pat. No. 2,040,121.
However, in for example a domestic shower heater, the electrical power consumed by the heater is large i.e. several kilowatts and it has been found that burst firing a heating means with such high power levels, leads to significant problems in suppression. The advantage of burst firing is that if there is any fluctuation in the flow rate and/or temperature of the inflowing water, such changes can easily be accommodated by sensing this flow rate/temperature change and changing the rate of burst firing or varying the "on" time relative to the "off" time to maintain the temperature of the outflowing water constant.
Another proposal has been for the electrical heating means to comprise a plurality of heating elements each of different heating value, and to switch the elements on and off in different combinations to achieve different heating levels. Such an arrangement is described in U.K. Pat. No. 2,007,046.
However, the disadvantage of such an arrangement is that any small fluctuations in flow rate and/or temperature of the inflowing water cannot easily be accommodated; it would be necessary on sensing such flow rate/temperature change, to switch on or off at least one of the heating elements, which could easily over compensate for the small change. Another disadvantage is that it would be necessary to provide a range of heating elements each of different value, and the tolerance of each element would have to be small so that small changes in temperature of the outflowing water can be achieved, as successive elements are switched on or off. This obviously increases costs.
The advantage of such an arrangement is that there are no suppression problems as with burst fired arrangements.
To minimise the effects of fluctuations in the pressure of inflowing water on the flow rate and temperature of the outflowing water, known water heaters of the kind specified have usually been provided with a pressure reducing valve, whereby the pressure of the water flowing through the heater once set has been maintained substantially constant. However again, such valves are expensive and in any case, do not operate efficiently.